C. P. Snow once remarked that not knowing the second law of thermodynamics is like never having read Shakespeare. Yet, while many people grasp the first law of energy, "Energy can neither be created nor destroyed," few recognize the second, "Entropy can only increase." What is entropy anyway, and why must it increase? Whether we want to know how a device as simple as a refrigerator works or understand the fate of the universe, we must start with the concepts of energy and entropy.

In The Refrigerator and the Universe, Martin and Inge Goldstein explain the laws of...